1. Field of the Invention
The present invention relates to the field of flow meters. In particular, the present invention relates to a method in hybrid flow meters.
2. State of the Art
Flow meters are using different technologies to measure the flow rate. Most flow meters do not measure the flow rate directly, but measure the velocity which is multiplied by the cross-sectional area to calculate the flow rate. In open channel flow measurements, some technologies measure the level and convert the level into flow rate using empirical equations such as the Manning equation, or convert the level into flow rate using mathematical equations when flumes or weirs are used. Other open channel technologies are using the continuity equation Q=A*V (flow rate Q equals area A multiplied by average velocity V). The level is measured and associated with the channel shape to calculate the wet area. The local velocity is measured using different technologies and converted to the average velocity of the wet cross-sectional area.
Depending on the application, one specific technology has advantages or disadvantages compared with the other. Not only the application, but the broad range of flow conditions, types of fluid or concentration of different materials present in the fluids to be measured, have different impact on the accuracy and measurement results depending on the measurement technology used.
Rain, storm and waste water channels, natural streams and man-made channels are mostly exposed to consistently changing conditions, i.e. low flows, high flows, surcharge flow conditions, reverse flow, stagnation and non-uniform velocity distribution profiles, as well as continuous variation of the fluid nature.
Current flow measurement technologies are only accurate within a prescribed range of conditions. If the condition changes, the stated or nominal accuracy of these flow meters is no longer valid. As an example, we can take the flume or weir with level meter which needs free discharge of water flow to stay within its +/−5% stated accuracy. This stated accuracy is totally lost once the primary device (flume or weir) is totally or partially submerged.
The submerged velocity/level sensors such as Doppler-pressure and EM-pressure sensor combinations have problems due to the built-up of silt, fouling sensors, catching debris, non uniform velocity distribution and variation of particle concentration. Additionally, low or dry day flow conditions will present measurement problems especially for pulsed cross-correlation or time-gated Doppler sensors due to the dead band, resulting in an amount of fluid level above the sensor in which no velocity measurement can be taken.
Surface detecting radar flow measuring devices solve the maintenance problem of fouled sensors and minimum fluid level to measure the velocity, but cannot measure during submerged flow conditions, when the radar velocity sensor is submerged, as an air to water interface is required for radar Doppler velocity sensors. Those flow meters also lose signal at velocities under 0.1 to 0.2 m/s.
In order to overcome these problems so-called “hybrid technology” flow meters have been designed. They use one technology, which in some cases is also referred to as the master technology, before they switch over to a different or secondary technology at the crossover point.
Some hybrid systems switching over from one measuring technology to another are using hysteresis at the crossover point to avoid constant back and forth jumping.
Present hybrid flow meters, switching from one technology to the other at a crossover point, are suffering from jumps on the flow output, because the technologies are different and, consequently, the flow measurement results under the same hydraulic flow conditions are different. Such jumps result in control and regulation problems in the downstream equipments (inappropriate control and regulation such as faulty valve positioning, inappropriate closing or opening of valves, incorrect pump control, etc.).
In order to avoid those jumps, one existing technology calibrates the secondary technology based on the readings from the master technology. This technique is experiencing accuracy problems under different specific flow situations, as the master technology might already be at its limits and, consequently, inaccurate when calibrating the slave technology.